Wave division multiplexing (WDM) optical networks are well known in the art. A WDM channel is typically transmitted by a single mode semiconductor laser. Information to be communicated is imposed on the light by modulating the laser current or by externally modulating the light by applying a voltage to a modulator coupled to the laser source. A receiver employs a photo-detector that converts the light into electric current. Typically, modulation may affect the amplitude of the light or the phase of the light, or both.
Regardless of whether signaling is based on phase-shift-keying (PSK), Amplitude-Shift Keying (ASK), or the simplest form of ASK, On-Off Keying (OOK), the most universal impairment of the optical signals is due to the accumulation of noise. Electrical noise can arise in the transmitter or especially in the receiver, but in optical communication networks covering long distances, the dominant source of noise is usually the buildup of Amplified Spontaneous Emission (ASE) from the optical amplifiers used to overcome transmission and switching losses. At the receiver end of an optical link, this spontaneous emission interacts with itself and with the optical signal to create electrical noise known as spontaneous-spontaneous beat noise or signal-spontaneous beat noise. The degradation due to ASE is often characterized by the Optical Signal-to-Noise Ratio (OSNR), defined as the ratio of the signal power in a single wavelength channel, divided by the noise power within a band of 0.1 nm, in the same wavelength channel.
Multipoint optical networks present a challenging environment for signal quality measurements, such as OSNR measurements. FIG. 1 shows a graph 100 of optical power density as a function of wavelength in a multipoint optical network. The noise level around a wavelength will vary, as shown by the height of the local noise levels 102. Additionally, for each WDM signal 104, the OSNR for each wavelength channel may be quite different from adjacent channels, due to the different path traversed by each wavelength channel through the network. Thus, in order to measure OSNR (or other measures of signal quality) in a multipoint WDM network, a narrow optical filter is typically used to select a single channel, or a part of a channel, to sample the signal quality of one optical wavelength at a time. The filter is a relatively expensive element, so it must be shared and tuned sequentially among a plurality of wavelengths to share its cost. Alternatively, a dispersive element may be used to direct wavelength channels to multiple photodetectors in parallel, adding to the photodetector cost.
Measuring OSNR should be possible by inspecting the time dependence of the signal-spontaneous (SSP) beat noise, provided the signal is known. FIGS. 2a through 2d show waveforms of photocurrent vs. time for an amplified optical link in which the signal is a simple on-off square wave. The received waveform as shown in FIG. 2a can be decomposed into the sum of the non-stochastic components (signal plus the time-averaged ASE power as an offset) as shown in FIG. 2b, the time-stationary noise components (i.e. thermal noise and spontaneous-spontaneous beat noise) as shown in FIG. 2c, and finally the noise components which switch on and off in time synchronously with the signal, comprising shot noise and signal-spontaneous beat noise as shown in FIG. 2d. For this example, it is assumed that the received power is reasonably high, thus the shot noise and thermal noise are negligible. FIG. 2d shows an obvious repetitive pattern at the period of the signal square wave, so it may be expected to observe a conspicuous feature in the Power Spectral Density (PSD) of the photocurrent at modulation frequency Δ.
A paper by Rossi, Dimmick, and Blumenthal (Journal of Lightwave Technology, vol. 18, pp. 1639-1648, and included herein by reference) discusses a scheme to extract OSNR values in WDM systems from the Carrier-to-Noise Ratio (CNR) of RF subcarriers. However, Rossi's method does not appear to work in a network where the noise level varies from one channel to another. Standard analysis of the CNR (see, for example, the book chapter by Mary Phillips and Ted Darcie ‘Lightwave Analog Video Transmission’, chapter 14 in Optical Fiber Telecommunications IIIA, Academic Press 1997, and included herein by reference) shows no frequency peak in the PSD of the SSP term, computing the SSP noise as a constant appropriate to the time-averaged signal power.
OSNR measurement also becomes extremely challenging as systems move towards higher spectral efficiency. As data rates per channel rise, the wavelength range occupied by each modulated signal increases. At the same time, systems are being designed with closer channel spacing, in order to pack more channels within the wavelength range accessible to practical optical amplifiers. Both of these developments tend to eliminate the well-defined noise shoulders framing each channel as shown in FIG. 1. In high spectral-efficiency systems, there may be no wavelengths where the noise can be measured separately from the signal, prohibiting the traditional method of interpolation from being used.
Thus, there exists a need for a method of signal quality measurement that can be implemented in WDM optical networks, providing rapid, accurate and low-cost measurements of multiple wavelength channels without requiring a narrow optical filter or a wavelength-dispersive element. There also exists a need for a method of OSNR measurement which can measure both noise and signal at exactly the same wavelength, without the need to interpolate from noise-only measurements of flat channel shoulders.